1. Field of the Invention
The present invention relates to a thick film paste which is used for the formation of a conductor, a resistor, an insulator, a protector or the like, in a ceramic wiring substrate by a screen printing process, and which is excellent in printability.
2. Description of Prior Art
A ceramic wiring substrate is produced by, for example, a process comprising screen printing a conductor paste for wiring on a fired ceramic or unfired green sheet substrate, further screen printing an insulating paste for an insulating layer thereon, drying, repeating the foregoing procedure, and firing. Beside the above conductor and insulating pastes, pastes for a resistor and a protector are applied according to necessity. These pastes are mainly used in a screen printing process, so that they are collectively called a thick film paste.
Generally, a thick film paste is prepared, for example, by adding conductive powder, such as metal powder, and insulating powder, such as ceramic or glass powder (for the preparation of a conductor paste), or an electrically resistive component, such as one based on RuO.sub.2 (for the preparation of a resistor paste), to a mixture of an organic resin and a solvent, which is called a vehicle. In the conventional vehicle, for example/ethyl cellulose or acrylic resin is used as an organic resin and butyl carbitol acetate, .alpha.-terpineol or .beta.-terpineol is used as a solvent.
For example, U.S. Pat. No. 4,394,171 (Japanese Patent Laid-Open No. 31509/1983) describes a thick film conductor composition containing a vehicle composed of ethyl cellulose (organic resin) and .beta.-terpineol (solvent).
The conventional thick film paste using the above vehicle has a drawback in that a viscosity increase occurs during printing to thereby affect the printability. The viscosity increase would result in rendering the separation of the paste from the printing plate unsuccessful to thereby cause pinholes and, in extreme cases, cause wire breakage in the conductor and short circuit in the insulating layer. The reason for the viscosity change is that the use of an organic solvent necessitates care for ventilation and exhaust during printing and drying and the solvent is evaporated to thereby bring about an increase in the paste viscosity during printing. The conventional thick film paste has another drawback in that bleeding sometimes occurs during printing. The occurrence of bleeding in a conductor paste for wiring deteriorates fine line quality. The cause of the bleeding is poor wettability of a solvent to conductive metal powders such as Ag, Au, Pd, Pt, etc.